Heat-curable products obtained by reacting compounds containing amine or imine groups (e.g., ureas, amides, aminotriazines, and the like) with aldehydes (e.g., formaldehyde, glyoxal, benzaldehyde, and the like) have been known for a number of years. Resins obtained by curing such condensation products possess an excellent combination of physical properties and are widely used in glues and molding compounds, as finishes for paper and textiles, and as surface coatings. The curable resins can be used per se or they can be further modified before curing, e.g., by alkylation with an alcohol such as methanol or butanol to provide solubility and compatibility and/or by admixture with other co-reactive materials such as polyfunctional compounds containing hydroxyl groups or carboxyl groups (e.g., glycols, alkyd resins, polyester resins, and the like). Broadly, the present invention is concerned with amino resins which are suitable for all conventional purposes. However, in its most preferred aspects, it is concerned with soluble forms or liquid forms of such amino resins, which are well known to be superior as coatings for metals and coatings or impregnants for cloth, paper, and the like. Such curable resins commonly comprise urea- or melamine-aldehyde condensation products or the reaction products thereof with alcohols (e.g., methylol ureas or methylol melamines) and alkylated derivatives thereof (e.g., methylated or butylated derivatives) either alone or dissolved in a suitable solvent. These specific amino resins are coated onto three-dimensional substrates (e.g., metal, glass, wood, plastics, and the like) and then heat-cured. The curing mechanism is condensation and crosslinking to split out water, alcohol, or formaldehyde. Curing can be effected without a catalyst if the resins are heated for a long enough time, i.e., for hours or days. However, for immediate curing or for curing at more moderate temperatures, an acid is often added to function as a crosslinking catalyst. The acidic catalysts previously used with amino resins include boric acid, phosphoric acids, acid sulfates, sulfonic acids and sulfonyl halides, hydrochlorides, ammonium phosphates and polyphosphates, acid salts of hexamethylene tetramine, phthalic acid, oxalic acid, and the like.
L. V. Gallagher U.S. Pat. No. 3,979,478 issued Sep. 7, 1976 discloses that high molecular weight polyalkylaromatic polysulfonic acids (e.g., dinonylnaphthalene disulfonic acid) are superior catalysts for curing amino resin systems. These acid catalysts cure the resins in short periods of time and produce products having superior physical properties. These acid catalysts can be stored in the form of thermally-decomposable adducts to provide greater shelf life.
M. M. White U.S. Pat. No. 3,474,054 issued Oct. 21, 1979 teaches that amine salts of aromatic sulfonic acids, preferably tertiary amine salts (e.g., pyridine salt of p-toluene sulfonic acid), can be utilized to cure amino resin coating compositions.
F. E. Tropp et al. U.S. Pat. No. 3,293,324 issued Dec. 20, 1966 discloses that the 2-dimethylamino-2-methyl-1-propanol salt of p-toluene sulfonic acid can also be utilized to cure thermosetting aminoplast resins.
L. J. Calbo U.S. Pat. No. 4,200,729 issued Apr. 27, 1980 and U.S. Pat. No. 4,251,665 issued Feb. 17, 1981 disclose particular adducts which impart outstanding resistance properties to the cured resins. Aromatic sulfonic acids in association with oxa-azacyclopentanes (e.g., 4,4-dimethyl-1-oxa-3-aza-cyclopentane) are outstanding latent catalysts for curing amino resin compositions. Compositions containing the latent catalysts have exceptional package stability and can be stored ready for use for relatively long periods of time without significantly detracting from the composition's usefulness. It is believed these advantageous results are attributable to a unique combination of the high volatility of the amine component and the low dissociation constant incident to the oxa-azacylopentane adducts. This combination, in addition to other factors, may provide for a particularly effective association of the adduct with the resin during the curing process and result in fast curing times and cured resins having superior properties. The cured resins have superior properties, particularly water resistance, compared to resins cured with unneutralized aromatic sulfonic acids and to other amine adducts of the aromatic sulfonic acids.